In the field of optical communication systems the source of an optical signal, such as a laser, is often coupled to a utilization device, such as an optical waveguide (integrated or fiber optic), a photodetector, a modulator, a fiber amplifier, an optical isolator, or a combination of such devices, by means of a lens arrangement. The latter may include a single lens or multiple lenses, and the lenses themselves may take on a variety of shapes such as spherical, aspherical, cylindrical, graded index (GRIN), etc. A typical design positions a spherical lens between the output face of the laser and the input face of an optical fiber, with these faces being located so as to efficiently couple light to and from the lens. Inasmuch as the position of these components relative to one another is critical, often measured in terms of a micrometer or less, one aspect of the prior art locates devices in etched precision features (e.g., grooves, cavities) formed by crystallographic planes in single crystal silicon substrates to meet the necessary alignment tolerances. This prior art technology is known as silicon optical bench (SiOB) technology. For an illustration of the use of spherical lenses disposed in pyramidal cavities, see, for example, U.S. Pat. No. 4,978,189 granted in 1990 to G. E. Blonder et al and assigned to the assignee hereof.
Of particular interest is the prior art SiOB confocal coupling arrangement shown in FIGS. 3&4. In this design a symmetrical, pyramidal cavity 3 is etched into the top major surface of a &lt;100&gt;-oriented Si substrate 1. The side walls of the cavity are formed by &lt;111&gt;-oriented crystallographic planes which make an angle of 54.7.degree. with the (100) plane. A spherical sapphire lens 5 is positioned in the cavity, and a semiconductor laser 7 is located on the top surface of the substrate, the output facet of the laser being located so as to efficiently couple light to the lens. Because the output beam of such a laser often has a wide divergence angle, efficient coupling requires that the laser be positioned in close proximity to the lens, i.e., at a distance not greater than the focal length of the lens. Because the focal point falls in the gap between the edge of the cavity and the surface of the lens, the laser typically overhangs the edge of the cavity in order to satisfy the focal length requirement. In many designs of this type as much as a quarter or a third of the laser length may overhang the edge. This overhang is a problem because the substrate is not just as a mechanical support; it is also a heat sink. Consequently, the overhanging portion of the laser may not be adequately thermally coupled to the substrate and may overheat, adversely affecting laser performance or lifetime.
Thus, a need remains in the art for a SiOB coupling arrangement which essentially eliminates the need for the laser to overhang the edge of the cavity and thus removes this source of unreliability.